Calibration of a spatial light modulator

ABSTRACT

A method for calibrating a spatial light modulator comprising an array of channels includes selecting a plurality of channel sets; operating each of the channel sets to provide corresponding output radiation; providing a detector for measuring the output radiation; determining a plurality of intensity values, each representing an intensity of the output radiation provided by a different one of the channel sets; providing a correction factor for each of the channels sets, wherein each correction factor remains constant during a subsequent recalibration of the spatial light modulator; modifying each determined intensity value in accordance with a corresponding one of the correction factors; determining a difference between one of the modified intensity values and a target intensity value; and reducing the determined difference by adjusting a control level of at least one channel in the channel set corresponding to the one of the modified intensity values.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of prior U.S. patent application Ser.No. 12/609,075, filed Oct. 30, 2009 (now U.S. Publication No.2011/0102780), which is hereby incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates to apparatus for forming images on a surface, andmore particularly to improvements in the calibration of a spatial lightmodulator employed by the apparatus.

BACKGROUND OF THE INVENTION

Spatial light modulators, also referred to as light valves, have founduse in many different fields. One particular industrial field in whichthese devices have been employed is the display industry. Another fieldwhere spatial light modulators have made a significant impact is theprinting industry, where they are extensively used with lasers to imagevarious recording media. Recording media can include various printingplates, printing sleeves, and printing cylinders for example. The lasersemployed in these applications often emit radiation having wavelengthssuitable for marking a sensitized surface of the recording media. Insome cases the lasers emit radiation comprising near-infrared orultraviolet wavelengths.

Spatial light modulators typically include a one or two-dimensionalarray of light valve channels. Each of the channels can be selectivelyoperated to provide an output radiation beam which can be used to form aunit element of an image typically referred to as an image pixel. Insome cases, an output radiation beam is provided by reflecting radiationfrom the spatial light modulator. In some cases, an output radiationbeam is provided by transmitting radiation through a spatial lightmodulator.

One particular subset of spatial light modulators is based on thereflection of incident radiation from micro-miniature deformablemirrors. Prior art deformable mirror light modulators can be generallydivided into several types. For example, cantilever or hinged mirrorlight modulators deflect radiation when bending or tilting the mirrorelements. A well-known example in this category is the digitalmicro-mirror device (DMD) technology developed by Texas InstrumentsIncorporated. Membrane light modulators employ a flat membrane that isdeformed into a concave or spherical mirror which focuses radiation.

Another subset of spatial light valves diffracts radiation by forming aperiodic physical pattern. A well-known example in this category is thegrating light valve developed by the Silicon Light Machines Corporationof Sunnyvale, Calif. Total internal reflection (TIR) spatial lightmodulators include an electro-optic material whose optical propertieschange in accordance with the strength of an electric field establishedwithin the material. Conventional TIR modulators typically include aplurality of electrodes that are arranged in an interdigitatedrelationship on a support surface of an electro-optic member. Othersurfaces of the member are arranged to cause input radiation to refractand undergo total internal reflection at the support surface. Upon theapplication of a suitable voltage to a corresponding one of theelectrode sets, an electric field is established in a portion of theelectro-optic member which alters the refractive index of the member andcauses the electrode set to behave in a manner similar to a diffractiongrating.

Spatial light modulators can require calibration for various reasons.For example, in imaging applications calibration may be required toalleviate image artifacts. Typically, there are a number of imagingparameters that need to be optimally set to achieve a desired qualityresult. One important parameter is the level of radiation exposureprovided on the recording media. Exposure is typically defined as theamount of radiant energy per unit area that impinges on the recordingmedia during the imaging process. Depending on the recording media, itmay be necessary to control this parameter to within a few percent orless. This situation is further compounded when multiple outputradiation beams are provided by a spatial light modulator. In this case,each beam needs to impart a substantially equal exposure on therecording media so that various artifacts including banding are notcreated.

Calibration of spatial light modulators can include balancing thevarious radiation beam intensities provided by an array of modulatorchannels in a process typically referred to as beam balancing. Beambalancing techniques attempt to establish a desired intensitydistribution (i.e. also referred to as intensity profile) across all theoutput radiation beams that can be provided by the channels of a spatiallight modulator. To achieve a desired intensity profile, one needs toknow with some degree of certainty how each image pixel changes inresponse to change in the control settings of a channel corresponding tothe image pixel and possibly, channels that neighbor the correspondingchannel.

Some conventional beam balancing methods have employed multi-valuedetectors to measure an intensity profile. Some conventional multi-valuedetectors typically include a plurality of detection elements whosenumber equal, or exceed the number of spatial modulator channels thatare activated to provide the detected output radiation beams. Radiationfrom each of a plurality of different sets of the modulator channels canbe simultaneously detected by multi-value detectors to provide a spatialdistribution of intensity values, each of the intensity valuescorresponding to the radiation provided by a different one of themodulator channel sets. In the limit, multi-value detectors can beemployed to determine an intensity profile across entirety of themodulator array on the basis of single channel resolutions. Examples ofmulti-value detectors include laser beam profilers that are diagnosticdevices that can measure the entirety of an intensity profile of asupplied radiation. Beam profilers can be used to accurately determine adetailed intensity profile shape of a plurality of radiation beams. Beamprofilers can include photo-sensor based beam profilers that comprisevisible or near-infrared CCD or CMOS sensors. Beam profilers can includescanning beam profilers that scan a beam profile with various pinholes,slits, or knife edges.

Despite their accuracy and resolution, many multi-value detectors can beconsidered to be prohibitively expensive if they are to be incorporatedinto a recording apparatus. To alleviate these costs issues, the use ofsingle-value detectors has been proposed for use in the detection ofoutput radiation beam intensity. Single-value detectors are simpler,less complicated, and less expensive than multi-level detectors. In asimilar fashion to multi-value detectors, single-value detectors cansimultaneously detect radiation from each of a plurality of modulatorchannels. However, single-value detectors can not distinguish betweenthe different portions of the radiation provided by each of themodulator channels. Consequently, single-value detectors provide only asingle intensity value representing the total radiation that isprovided. The data determined by using a single-value detector does notcontain any information on how the radiation intensity is spatiallydistributed. In particular, it does not indicate how much energy eachimage pixel would receive during exposure.

To overcome this shortcoming, single-value detectors can be employed toprovide an intensity profile for all the operable channels in a spatiallight modulator by dividing all the modulator channels into sets andindividually activating each set to provide corresponding radiationwhich is separately measured by the detector. An intensity value isseparately determined for each of the channel sets and an intensityprofile is generated by mapping each of the separately determinedintensity values with positional information of a corresponding one ofthe channel sets. For example, a portion of the intensity profile can begenerated by measuring the total intensity of radiation provided by afirst one of the channel sets while the remaining channels are turnedoff. Repeating this measurement for each of a sequence of differentchannels sets making up the remainder of the spatial light modulatorprovides a set of intensity values representing the intensity profile.

The number of channels employed in each of the channels sets during thisprocess is typically based on several factors. For example, channel setscomprising only a few channels each can provide a suitable granularityfor making effective corrections to intensity deviations highlighted bya subsequently determined intensity profile. However, larger numbers ofthese channel sets having fewer channels would be required to completethe intensity profile thereby increasing the calibration time. Thepresent inventors have additionally determined that the number ofchannels employed in each channel set also has an effect on the accuracyof the intensity value measured by a single-value detector. For example,FIG. 1 shows a plot comparing various intensity profiles for a recordinghead produced by the Eastman Kodak Company. In this particular case, therecording head employs a spatial light modulator having 896 channels.Three different intensity profiles are illustrated in accordance withthe KEY of the FIG. 1 plot. The various intensity values are shown inarbitrary units. A multi-value detector intensity profile 450 acts as abase-line to compare the accuracy of a first single-value detectorintensity profile 460 and a second single-value detector intensityprofile 470. Each of the multi-value detector intensity profile 450, thefirst single-value detector intensity profile 460 and the secondsingle-value detector intensity profile 470 have been “smoothed” forclarity and therefore do not show scatter among individual intensityvalue data points that each of the profiles was generated from.

Multi-value detector intensity profile 450 represents a condition whereeach of the channels in the array has been balanced using a multi-valuedetector (i.e. a laser beam profiler) in a manner similar to thatpreviously described. In this case, the intensity level of variouschannels was determined using the multi-value detector, and controllevels of each of the channels were adjusted to balance the channels toproduce the substantially level multi-value detector intensity profile450. Each of the first single-value detector intensity profile 460 andthe second single-value detector intensity 470 were generated with theuse of a single-value detector. Specifically, after the spatial lightmodulator channels were balanced using the multi-value detector, theintensities of different sets of the balanced channels were measuredusing the single-value detector in a manner similar to that previouslydescribed. A plurality of first channel sets, each comprising thirty two(32) channels was used to generate the first single-value detectorintensity profile 460 while a plurality of second channel sets, eachcomprising three (3) channels was used to generate the secondsingle-value detector intensity profile 470.

The FIG. 1 plot shows that despite having accurately balanced thespatial light modulator channels using a multi-value detector, each ofthe first single-value detector intensity profile 460 and the secondsingle-value detector intensity profile 470 show deviations from thisbalanced condition. In this regard, each of the first single-valuedetector intensity profile 460 and the second single-value detectorintensity profile 470 is distorted. The first single-value detectorintensity profile 460 that was generated using the first channel setscomprising thirty two (32) channels is shown deviating by about 1% fromthe uniform multi-value detector intensity profile 450 while the secondsingle-value detector intensity profile 470 that was generated usingsecond channel sets comprising three (3) channels shows as much as 4%deviation. Although they do not wish to be bound by any particulartheory, the present inventor believes that due to the details of theoperation of the spatial light modulator and the propagation of theradiation in the recording head, an intensity profile generated using asingle-value detector will typically deviate from an intensity profilegenerated with a multi-value detector. The deviation magnitude dependson the number of channels in the detected channel sets, with strongerdeviations resulting from channel sets having fewer numbers of channels.

There is a need to provide improved methods and systems for calibratinga spatial light modulator. There is a further need to provide improvedmethods and systems for reducing deviations in an intensity profilegenerated for a spatial light modulator using a single-value detector.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a method forcalibrating a spatial light modulator comprising an array ofindividually addressable channels includes selecting a plurality ofchannel sets from the array of channels; operating each of the channelsets to provide corresponding output radiation; providing a detector formeasuring the output radiation provided by each of the channels sets;determining a plurality of intensity values, wherein each intensityvalue is a single value representing an intensity of the outputradiation provided by a different one of the channel sets; providing acorrection factor for each of the channels sets, wherein each correctionfactor remains constant during a subsequent recalibration of the spatiallight modulator; modifying each determined intensity value in accordancewith a corresponding one of the correction factors; determining adifference between one of the modified intensity values and a targetintensity value; and reducing the determined difference by adjusting acontrol level of at least one channel in the channel set correspondingto the one of the modified intensity values.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments and applications of the invention are illustrated by theattached non-limiting drawings. The attached drawings are for purposesof illustrating the concepts of the invention and may not be to scale.

FIG. 1 shows a plot comparing various single-value detector intensityprofiles with a multi-value detector intensity profile;

FIG. 2 schematically shows a recording apparatus for forming an image ona recording media as employed in an example embodiment of the invention;

FIG. 3 schematically shows an optical system employed in an exampleembodiment of the invention;

FIG. 4A schematically shows a plan view of a spatial light modulatoremployed by an example embodiment of the invention;

FIG. 4B schematically shows a side view of the spatial light modulatorof FIG. 4A;

FIG. 5 shows a block diagram representing a method for calibrating aspatial light modulator as per an example embodiment of the invention;

FIG. 6 schematically shows a plurality of different channel setsselected from a spatial light modulator in accordance with an exampleembodiment of the invention;

FIG. 7 shows a first intensity profile generated from determined firstintensity values as per an example embodiment of the invention;

FIG. 8 shows a second intensity profile generated from determined secondintensity values as per an example embodiment of the invention; and

FIG. 9 schematically represents a plurality of second channel setsselected in accordance with an example embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following description specific details are presented toprovide a more thorough understanding to persons skilled in the art.However, well-known elements may not have been shown or described indetail to avoid unnecessarily obscuring the disclosure. Accordingly, thedescription and drawings are to be regarded in an illustrative, ratherthan a restrictive sense.

FIG. 2 schematically shows a recording apparatus 10 for forming an image19 (i.e. schematically represented by broken lines) on a recording media17 as employed in an example embodiment of the invention. Recordingmedia 17 can include various media comprising a surface suitable forforming image 19 thereupon. Recording apparatus 10 includes a mediasupport 12, which in this example embodiment is configured as per anexternal drum configuration. Other embodiments of the invention caninclude other forms of media supports including internal drum andflat-bed configurations for example.

In this example embodiment, recording media 17 is supported on acylindrical surface 13 of media support 12. One or more edge portions ofrecording media 17 are secured to cylindrical surface 13 by clamps 28.In other example embodiments, recording media 17 can be secured to mediasupport 12 by other methods. For example, a surface of recording media17 can be secured to cylindrical surface 13 by various methods includingproviding a low-pressure source between the surfaces. In various exampleembodiments, media support 12 is movably coupled to support 20. In thisexample embodiment, media support 12 is rotationally coupled to support20. In this example embodiment, media support 12 includes a plurality ofregistration features 25. Registration features 25 are employed toorient recording media 17 with respect to media support 12 in a desiredorientation.

Recording apparatus 10 includes recording head 16, which is movablerelative to media support 12. In this example embodiment of theinvention, media support 12 is adapted to move by rotating about itsrotational axis. In this example embodiment, recording head 16 ismounted on movable carriage 18. Carriage 18 is operated to causerecording head 16 to be moved along a path aligned with the rotationalaxis of media support 12. Motion system 22 is employed to providerelative movement between recording head 16 and media support 12. Motionsystem 22 (which can include one or more motion systems) can include anysuitable drives needed for the required movement. In this exampleembodiment of the invention, motion system 22 is used to move mediasupport 12 along a path aligned with main-scan axis MSA and is used tomove recording head 16 along a path aligned with sub-scan axis SSA.Guide system 32 is used to guide carriage 18 which is moved under theinfluence of transmission member 33. In this example embodiment of theinvention, transmission member 33 includes a precision screw mechanism.In some example embodiments, several recording heads 16 are moved in amanner where each of the recording heads 16 is moved independently ofone another. In some example embodiments, several recording heads 16 aremoved in tandem.

Those skilled in the art will realize that various forms of relativemovement between recording head 16 and media support 12 can be used inaccordance with the present invention. For example, in some casesrecording head 16 can be stationary while media support 12 is moved. Inother cases, media support 12 is stationary and recording head 16 ismoved. In still other cases, both the recording head 16 and the mediasupport 12 are moved. One or both of recording head 16 and media support12 can reciprocate along corresponding paths. Separate motion systemscan also be used to operate different systems within recording apparatus10.

Controller 30, which can include one or more controllers is used tocontrol one or more systems of recording apparatus 10 including, but notlimited to, various motion systems 22 used by media support 12 andcarriage 18. Controller 30 can also control media handling mechanismsthat can initiate the loading or unloading of recording media 17 to orfrom media support 12 respectively. Controller 30 can also controlrecording head 16 to form image 19 in accordance with image data 37.Various systems can be controlled using various control signals orimplementing various methods. Controller 30 is programmable and can beconfigured to execute suitable software and can include one or more dataprocessors, together with suitable hardware, including by way ofnon-limiting example: accessible memory, logic circuitry, drivers,amplifiers, A/D and D/A converters, input/output ports and the like.Controller 30 can comprise, without limitation, a microprocessor, acomputer-on-a-chip, the CPU of a computer or any other suitablemicrocontroller. Controller 30 can consist of several different orlogical units, each of which is dedicated to performing a particulartask in various example embodiments of the invention.

In this example embodiment, recording head 16 is adapted for directingoutput radiation towards recording media 17. The wavelength of theoutput radiation is selected to suit the type of recording media 17 thatis being imaged and can include wavelengths in the infrared, visible andultraviolet spectrums for example.

In this illustrated example embodiment, recording head 16 iscontrollable to emit various output radiation beams 121 while scanningover recording media 17 to form image 19. Output radiation beams 121 canbe image-wise modulated according to image data 37 specifying the imageto be written. Each output radiation beam 121 is controllable to form aunit element of image typically referred to as an image pixel onrecording media 17 in accordance with information provided by image data37. Various image pixels can be combined with other image pixels to formvarious features of image 19. In various example embodiments of theinvention, image pixels can be arranged in various image pixel patternsincluding halftone patterns, stochastic patterns and hybrid patterns forexample.

Image 19 can be formed on recording media 17 by different methods. Forexample, recording media 17 can include a modifiable surface, wherein aproperty or characteristic of the modifiable surface is changed whenirradiated by an output radiation beam 121. An output radiation beam 121can be used to ablate a surface of recording media 17 to form an image19. An output radiation beam 121 can be used to facilitate a transfer ofan image forming material to a surface of recording media 17 to formimage 19 (e.g. a thermal transfer process). An output radiation beam 121can undergo a direct path from a radiation source to the recording media17, or can be deflected by one or more optical elements towards therecording media 17.

In many cases, image 19 is formed by merging multiple sub-imagestogether, each of the sub images being formed during a correspondingmarking operation. The sub-images can be formed in different manners.For example, image 19 can be formed from plurality of markings referredto as “shots.” During each shot, recording head 16 is positionedrelative to a region of recording media 17. Once positioned, recordinghead 16 is activated to form an arrangement of image pixels on theregion of recording media 17. Once the arrangement of image pixels isformed, relative movement between recording head 16 and recording media17 is effected to position the recording head 16 in the vicinity of anadjacent region and another shot is taken to form a next image pixelarrangement.

The various sub-images can also be formed by scanning. In some exampleembodiments of the invention, scanning can be performed by deflectingoutput radiation beams 121 emitted by recording head 16 relative torecording media 17. In some example embodiments, scanning can includeestablishing relative movement between the recording head 16 andrecording media 17 as recording head 16 is activated to formcorresponding image pixels. In these example embodiments, columns ofimage pixels are formed along a scan direction as relative movementbetween recording head 16 and the recording media 17 is established.Relative movement can include moving one or both of the recording head16 and recording media 17. Each of the scanned image pixel columns arecombined to form a sub-image typically referred to as an image swath.

Different scanning techniques can be employed to form image swaths. Forexample, “circular” scanning techniques can be used to form “ring-like”or “circular” image swaths. A circular image swath can be formed whencontroller 30 causes recording head 16 to emit output radiation beams121 while maintaining recording head 16 at a first position alongsub-scan axis SSA and while moving media support 12 along a direction ofmain-scan axis MSA. In this regard, scanning occurs solely along amain-scan direction. After the completion of a first circular imageswath, recording head 16 is moved to a second position along sub-scanaxis SSA. A second circular image swath is then formed as recording head16 is operated to emit output radiation beams 121 while maintainingrecording head 16 at second position and while moving media support 12along a direction of main-scan axis MSA.

Helical scanning techniques can be employed to form helical image swathswhich are formed in a spiral or helical fashion over a surface ofrecording media 17. For example, helical image swaths can be formed whencontroller 30 causes recording head 16 to emit output radiation beams121 while simultaneously causing recording head 16 to move along adirection of sub-scan axis SSA and media support 12 to move along adirection of main-scan axis MSA. In this regard, scanning occurs alongboth a main-scan direction and along a sub-scan direction and eachhelical image swath comprises an orientation that is skewed relative tomain-scan axis MSA.

It is to be noted that other forms of skewed scanning techniques similarto helical scanning techniques can be used in various embodiments of thepresent invention. Skewed scanning techniques need not be limited toexternal drum configurations but can also be employed with otherconfigurations of recording apparatus. For example, in some internaldrum recording apparatus, media is positioned on a concave surface of amedia support while a radiation beam is directed towards an opticaldeflector positioned along a central axis of the media support. Theoptical deflector is rotated while moving along central axis to causethe radiation beam to follow a spiral path on the surface of therecording media. Flat-bed recording devices can include coordinatedmovement between a recording head and the recording media to formvarious image swaths with a particularly desired orientation.

FIG. 3 schematically shows an optical system 100 employed by recordinghead 16 as per an example embodiment of the invention. Optical system100 includes an illumination source 102 which can include a laser forexample and a spatial light modulator 200. Suitable lasers can includelaser diode arrays which are relatively easy to modulate, and haverelatively small size and low cost. The choice of illumination source102 can be motivated by various properties of recording media 17. One ormore optical elements 110 are positioned along the path of radiation 125emitted by illumination source 102 towards spatial light modulator 200.Optical elements 110 can include one or more lenses employed tocondition radiation 125 in various ways. For example, when diode laserarrays are employed, various degrees of beam divergence can exist alongeach of a plurality of different directions. Beam divergence can includefast axis divergence and slow axis divergence for example. Opticalelements 110 can include various lenses such as micro-lenses or crossedcylindrical lenses that are adapted to correct for these divergences.Optical elements 110 can include various elements adapted to mix orreflect radiation 125 such as light pipes and fly's eye integrators forexample. Optical elements 110 can include various lenses adapted tofocus or redirect radiation 125 emitted by illumination source 102.

Radiation 125 that is directed onto spatial light modulator 200 ismodulated in accordance with controller 30 which selectively controlsvarious individually addressable channels 210 (i.e. schematicallyrepresented in broken lines) of spatial light modulator 200 to formvarious radiation beams. In this example embodiment, channels 210 arearranged in a one dimensional array. In other example embodiments,channels 210 can be arranged in two dimensional arrays. Image data 37 isemployed by controller 30 to generate various output radiation beams 121which are directed along a path towards an imageable surface of arecording media 17 to form various image pixels 140 thereon. Otherradiations beams (not shown) that are not required by the formation ofvarious image pixels 140 are directed elsewhere. In this regard controllevels of each channel 210 are altered in accordance with a desire toform or not form an output radiation beam 121.

In this illustrated embodiment, the output radiation beams 121 requiredto form image pixels 140 pass through an aperture 150 while radiationbeams not required to form image pixels 140 (i.e. again not shown) areobstructed by aperture 150. One or more lenses (not shown) may beemployed to direct radiation from spatial light modulator 200 towardsaperture 150. One or more optical elements 170 are employed to directvarious output radiation beams 121 onto the imageable surface ofrecording media 17. Various other embodiments of the invention need notemploy aperture 150, and radiation beams not required by the formationof various image pixels 140 may fall by design outside the entrancepupil of a lens of optical elements 170.

FIGS. 4A and 4B schematically show respective plan and side views of aspatial light modulator 200 employed by an example embodiment of theinvention. In this example embodiment of the invention, spatial lightmodulator 200 is a total internal reflection (TIR) spatial lightmodulator. Spatial light modulator 200 comprises a member 212 whichincludes an electro-optic material and a plurality of electrodes 215 and216 arranged in an interdigitated relationship on a surface 218 ofmember 212. Member 212 includes surfaces 220 and 222 which are arrangedto cause radiation 125 to refract and undergo total internal reflectionat surface 218.

The various electrodes 215 and 216 are grouped into electrode groups S₁,S₂, S₃, S₄ . . . S_(n) which are collectively referred to as electrodegroups S. Each of the electrode groups S corresponds to a channel 210 ofspatial modulator 200. Each of the electrodes 215 in each of the groupsare coupled together and driven with corresponding one of individuallyaddressable voltages sources V₁, V₂, V₃, V₄ . . . V_(n) which arecollectively referred to as voltage sources V. Each of the individuallyaddressable voltages sources V is employed to alter the control levelsof a corresponding channel 210 in accordance with various image data 37signals. To simplify interconnect and driver requirements, allelectrodes 216 are interconnected to a common source (e.g. a groundpotential). In this case, electrodes 216 are coupled in a serpentinefashion among all the electrode groups S. In other example embodiments,the electrodes 216 in each of the electrode groups S are driven with oneof a plurality of individually addressable voltage sources (not shown)as described in commonly assigned U.S. patent application No. Ser.12/183,094 which is herein incorporated by reference.

Upon the application of a suitable control voltage by one of the voltagesources V₁, V₂, V₃, V₄ . . . V_(n) to a corresponding one of theelectrode groups S₁, S₂, S₃, S₄ . . . S_(n), an electric field isestablished in a region of the electro-optic material corresponding to achannel 210. The application of the voltage alters the refractive indexof the electro-optic material, thereby changing a birefringent state ofthe region. Under the application the corresponding drive voltage, thearrangement of electrodes 215 and 216 in each of the electrode groupsS₁, S₂, S₃, S₄ . . . S_(n) causes each of the electrode groups to behavein a manner similar to a diffraction grating. A birefringent state ofthe each of the regions can therefore be changed in accordance with theselective application of various voltages by corresponding voltagesources V₁, V₂, V₃, V₄ . . . V_(n). For example, in this case when novoltage is applied to a particular electrode group S, the correspondingchannel 210 assumes a first birefringent state in which an outputradiation beam 121 is provided from surface 222 and is directed towardsa surface of a recording media 17 to form an image pixel 140 thereon. Inthe case when a suitable voltage is applied to a particular electrodegroup S, the corresponding channel 210 assumes a second birefringentstate in which radiation is provided from surface 222 in a diffractedform which can be blocked by an obstruction such as aperture 150 to notform an image pixel 140.

In various example embodiments of the invention, control voltages areselectively imposed on each of the electrode groups S in accordance witha desired activation state of a channel 210 associated with each of theelectrode groups S. Activation states can include for example: an ONstate in which a channel 210 is activated to form an image pixel 140 onrecording media 17 and an OFF state in which a channel 210 is activatedto not to form a corresponding image pixel 140 on recording media 17. Itis to be noted that some leakage effects may be present and some amountof radiation may be directed towards recording media 17, even when aparticular channel 210 is activated with an OFF state.

In various example embodiments of the invention, the control levels of agiven channel 210 can be adjusted to cause different birefringent statesto be imposed in the electro-optic material associated with the channel210 such that various degrees of diffraction are established for each ofthe states. Different birefringent states can be used to adjust theintensity of output radiation beams 121 provided by a correspondingchannel 210. In this regard, different channels 210 can be attenuated todifferent levels in accordance with the degree of diffraction that isestablished in each of the channels 210. A given channel 210 can beattenuated to a desired level by directing a portion of the radiationprovided by the channel towards an obstruction such as aperture 150thereby blocking it from reaching the surface of recording media 17 anddirecting another portion of the radiation provided by the channel toform an output radiation beam 121. It is understood that other types ofspatial light modulators 200 can be employed and the present inventionis not limited to TIR spatial light modulators. Attenuation method ofvarious channels 210 in these other spatial light modulators 200 canvary in accordance with the particular architecture of each modulator.

In this example embodiment, detector 180 is provided for detectingradiation provided by various sets of channels 210. Detector 180 caninclude various sensors including various photo-sensors for example. Inthis example embodiment, detector 180 is a single-value detector.Detector 180 can include a large area photodiode by way of example. Inthis example embodiment, detector 180 is capable of detecting radiationfrom various combinations of output radiation beams 121 provided byspatial light modulator 200. In conjunction with controller 30, detector180 can be used to determine a single intensity value for each of theprovided combinations of output radiation beams 121. In this exampleembodiment, detector 180 is positioned to receive radiation provided byspatial light modulator 200 after the radiation has been conditioned byaperture 150. In this example embodiment, detector 180 is positioned tointersect radiation provided by spatial light modulator 200 at alocation upstream of a final lens of optical elements 170. In someexample embodiments, detector 180 is not located within recording head16. For example, detector 180 can be positioned at a location on support20 that can be irradiated by output radiation beams 121. Detector 180can also be positioned on movable media support 12, but additionalcommunications complications between detector 180 and controller 30 mayneed to be addressed in this configuration.

In this example embodiment, detector 180 is movable from a non-samplingposition 182 which does not intersect a path of travel of outputradiation beams 121, to a sampling position 184 which is along a path oftravel of output radiation beams 121. Detector 180 is shown in brokenlines at sampling position 184. An actuator system 185 is used toposition detector 180 between the non-sampling position 182 and thesampling position 184. Actuator system 185 can include various suitabledrives (e.g. electric motors) and guide systems. In this exampleembodiment, radiation provided by spatial light modulator 200 isdetected when recording head 16 is not employed to form image 19 onrecording media 17. Controller 30 can be programmed to operate detector180 on a predetermined schedule. Additionally, or alternatively,detector 180 can be operated to detect radiation provided by spatiallight modulator 200 in an “on-demand” fashion as requested by anoperator via a suitable user interface.

Once positioned in the sampling position, detector 180 measuressubstantially all of the total intensity of the radiation that passesthrough aperture 150. Detector 180 can be physically removed from theoptical path once the measurement is taken. In this example embodiment,detector 180 provides a single intensity value representing the totalintensity of the output radiation that would emerge at the output ofrecording head 16 if detector 180 had been located at the non-samplinglocation 182. In this example embodiment, data provided by detector 180does not contain any information on how the radiation intensity isspatially distributed. In this example embodiment, detector 180 cannottell how much energy would be received by each image pixel that could beformed by a channel set comprising multiple channels 210.

In some example embodiments, detector 180 need not detect the entiretyof each output radiation beam 121 that is provided by spatial lightmodulator 200. For example, a beam splitter (not shown) can be employedto provide a predetermined portion of each output radiation beam 121 todetector 180 while allowing remaining portions of each output radiationbeam 121 to travel along other paths. Those skilled in the art willrealize that the present invention can employ various methods to directoutput radiation beams 121 from spatial light modulator 200 to detector180.

FIG. 5 shows a block diagram representing a method 300 for calibrating aspatial light modulator 200 in accordance with an example embodiment ofthe invention. In this example embodiment, the calibration includes beambalancing the output radiation beams 121 provided by spatial lightmodulator 200. Various steps of method 300 are described with referenceto recording apparatus 10 and corresponding optical system 100respectively illustrated in FIGS. 2 and 3. This is for illustrationpurposes only, and other suitable recording apparatus can be employed inother example embodiments of the invention. Method 300 additionallyrefers to various sets of channels 210 selected from spatial modulator200. One arrangement of the various sets of channels 210 employed in anexample embodiment of the invention is schematically represented in FIG.6.

In step 310, the various sets of channels 210 are selected from spatiallight modulator 200. Specifically, a plurality of first channel sets 500is selected from the array of channels 210. In this example embodiment,spatial light modulator 200 includes N channels 210, from which aplurality of first channel sets 500 numbering Q is selected. In thisexample embodiment, each of the first channel sets 500 includes an equalnumber of channels 210 numbering X. In various example embodiments, eachchannel 210 in the array is part of at least one of the first channelsets 500. In this example embodiment, each channel 210 is part of onlyone of the first channel sets 500.

In this example embodiment, the number of channels 210 in each of thefirst channel sets 500 is selected to limit the size of distortions inan intensity profile that is to be subsequently generated by employingdetector 180 to measure output radiation provided by each of the firstchannels sets 500. As previously described, distortion in an intensityprofile generated by a single-value detector can be reduced by employingchannels sets having relatively large numbers of channels 210. Forexample, for the previously described recording head 16 comprising atotal of 896 channels as referenced in FIG. 1, first channel sets 500comprising thirty (32) channels 210 would typically be associated withsmaller intensity profile distortions than first channel sets 500comprising only three (3) channels 210.

In step 320, a plurality of first intensity values is determined. Eachof the first intensity values corresponds to radiation provided by oneof the first channel sets 500. Each of the first channel sets 500 isseparately operated to provide output radiation. In this exampleembodiment, all of the X channels 210 in a given first channel set 500are operated in accordance with substantially the same control levels.During the operation of a given first channel set 500, the totalintensity of corresponding outputted radiation is measured by detector180 while the control levels of all of the other channels 210 belongingto the other first channel sets 500 are set to maximum attenuation. Acorresponding set of measured intensity levels I_(M) is thus determinedfor the plurality of first channel sets 500. This procedure neglectscontributions from the channels 210 that were set to maximum attenuation(i.e. turned “OFF”). In practice, contribution to the intensity of agiven output radiation beam 121 from a fully attenuated channel 210 issmall, but not zero due to leakage effects. In some cases, the totalcontribution from all the turned “OFF” channels 210 may even exceed thecontribution from the operated first channel set if N>>X as is often thecase. In some example embodiments, this problem is alleviated by firstsetting the control levels of all N channels 210 to the maximumattenuation state so that most of the radiation that reaches spatiallight modulator at any location is diffracted and blocked by aperture150. Since the diffraction is not perfect, a small amount of radiationwill not be diffracted and will thus pass through aperture 150. Detector180 is then employed to measure the total intensity I₀ of thisnon-diffracted radiation. Each of the first intensity values can then beprovided by determining an intensity difference ΔI for each of the firstchannel sets, where ΔI=I_(M)−I₀.

In step 330, a first intensity profile is generated from the firstintensity values. To do this one needs to establish the correspondencebetween the particular locations in the intensity profile and thevarious first channels sets 500. In this example embodiment, the firstintensity profile is generated by plotting each intensity value as afunction of the position of the corresponding first channel set 500 inthe array of channels 210. FIG. 7 shows an example of a first intensityprofile generated for first channels sets 500, each comprising thirtytwo (32) channels 210. The various first intensity values are shown inarbitrary units. For comparison purposes, another intensity profile ofthe same channels 210 as provided by a multi-value detector (i.e. a beamprofiler) is shown. Both intensity profiles are identified as per theKEY in FIG. 7. The intensity profile corresponding to the multi-valuedetector represents a condition where various ones of the channels 210in the array were balanced using a beam profiler to provide asubstantially flat intensity profile. Once balanced using the beamprofiler, the channels 210 were grouped into the various first channelsets 500 and corresponding first intensity values were determined foreach of the first channel sets 500 using detector 180. FIG. 7 showsdeviations between the first intensity profile and the multi-valuedetector profile. In this regard, the first intensity profile isdistorted. However, since a relatively large number of channels 210(i.e. 32 channels) have been employed in each of the first channels sets500, differences among the various measured first intensity values arerelatively small.

In step 340, a channel set adjustment is performed to reduce adetermined difference between at least one of the first intensity valuesand a first target intensity value. In some example embodiment the firsttarget intensity value is equal to one of the first intensity values. Insome example embodiments, the first target intensity value is equal to,or less than, a minimum one of the determined first intensity values.Any determined differences between each of the at least one of the firstintensity values and the first target intensity value are reduced byadjusting control levels of a group of the channels 210 in spatial lightmodulator 200. In this example embodiment, a difference between a givenfirst intensity value and the first target intensity value is reduced byappropriately adjusting the control level of each of the X channels 210in a corresponding one of the first channels sets 500 in a directionappropriate for reducing the determined difference. In this exampleembodiment, each of the X channels 210 in an adjusted first channel set500 is adjusted by the same amount. In this example embodiment, channelcontrol levels in one or more of the first channels sets 500 areadjusted to cause each of the first channels sets 500 to havesubstantially equal corresponding first intensity values if re-measuredby detector 180. In this example embodiment, channel control levels inone or more of the first channels sets 500 are adjusted so that adjustedcontrol levels form a base line for a subsequent calibration step.Adjustment of the control levels of the channels 210 in a given one ofthe first channel sets 500 can include adjusting an attenuation level ofthe channels 210.

In step 350, a plurality of second channels sets 510 numbering R isselected from the array of channels 210. Each of the R second channelsets 510 includes Y channels 210. In this example embodiment, eachchannel 210 in spatial light modulator 200 is part of at least one ofthe second channel sets 510. In some example embodiments, each channel210 is part of only one of the second channel sets 510. In some exampleembodiments, the second channel sets 510 form an ordered sequence withinthe array of channels 210 and each second channel set 510 is selected inaccordance with its position in the ordered sequence. In some exampleembodiments, each of the second channels sets 510 is selected randomlyfrom the array of channels 210.

As previously described, relatively large deviations can arise in anintensity profile generated by a single-value detector when channelssets having relatively few channels are employed. This effect is morepredominate for channel sets having fewer numbers of channels 210. Inthe extreme, this effect can lead to considerable difficulty indetermining an intensity value for a channel set comprising a singlechannel. Nonetheless, this effect needs to be compensated for. In thisexample embodiment, the selected number of channels 210 in each of thesecond channel sets 510 causes relatively large deviations in anintensity profile that is to be subsequently generated from intensitymeasurements of radiation provided by the second channels sets 510. Inthis example embodiment, deviations associated with the intensityprofile corresponding to the second channel sets 510 are typicallylarger than deviations associated with the intensity profilecorresponding to the first channel sets 500. The number of channels 210employed in a given second channel set 510 can be limited by the minimumnumber of channels 210 required to provide output radiation that isdetectable by detector 180.

In this example embodiment, the number of channels 210 that is selectedin each of the second channels sets (i.e. Y) is less than the number ofchannels that was selected in each of the first channel sets (i.e. X).In some example embodiments, the number Y is less than the number X by afactor of 8 or more. In some example embodiments of the invention, thenumber Y is less than the number X by a factor of 10 or more. In thisexample embodiment, each of the second channel sets 510 comprises asufficient number of channels 210 to allow corresponding outputradiation to be detected by detector 180. The number of channels 210employed in each of the second channels sets 510 can be selected invarious manners, including direct experimentation. The present inventorshave employed second channels sets 510 comprising three (3) channels 210in some calibrations routines.

In this example embodiment, each of the second channels sets 510comprises channels 210 from one or more of the first channel sets 500.For example, a second channel set 510 (e.g. second channel set 510A) canbe selected in its entirety from a single first channel set 500, ordifferent portions of the second channel set 510 (e.g. second channelset 510B) can be selected from different first channels sets 500 such astwo adjacent first channel sets 500. In this example embodiment, variousones of the second channel sets 510 are subsets of a first channel set500. In this example embodiment, each of the second channel sets cancomprise channels 210 that were selected from one or more first channelsets 500 whose control levels were adjusted as per step 340.

In step 360, a plurality of second intensity values is determined, eachof the second intensity values corresponding to radiation provided by adifferent one of the second channel sets 510. In this exampleembodiment, detector 180 is employed to determine each second intensityvalue. In this example embodiment, each second intensity value is asingle value representing the intensity of the combined radiationprovided by all the channels 210 within a given second channel set 510.In this example embodiment, each of the second channel sets 510 isseparately operated to provide output radiation. In this exampleembodiment, each of the channels 210 in an operated second channel set510 is operated in accordance with control levels that were previouslyset for these channels to reduce differences between various ones of thedetermined first intensity values and the first target intensity value.Accordingly, various channels 210 in a second channels set 510 are nowoperated in accordance with previously adjusted control levels.

In step 370, a second intensity profile is generated from the determinedsecond intensity values. FIG. 8 shows a second intensity profilegenerated from determined second intensity values, each second intensityvalue corresponding to radiation provided by a second channel set 510having three (3) channels 210. The second intensity values as well asseveral mathematical curves that are fitted to the determined secondintensity values are identified as per the KEY in FIG. 8. The secondintensity profile was generated using a recording head 16 comprisingN=896 total channels 210. First channels sets 500, each comprisingthirty two (32) channels 210 were previously balanced in a mannersimilar to that taught by previously described steps 310, 320, 330, and340. Second channel sets 510, each comprising three (3) channels 210were selected from the balanced first channel sets 500. FIG. 8 showsthat distortions exist in the second intensity profile. Thesedistortions exist despite the fact that various channels 210 in thesecond channels sets 510 were previously adjusted to balance the thirtytwo (32) channel first channel sets 500. Although scatter exists amongvarious individual ones of the measured second intensity values, thesecond intensity profile generated by the entirety of the measuredsecond intensity values is distorted in a characteristic S-shapedmanner.

It has been determined that the S-shaped distortion in the secondintensity profile represents a systematic error of the measurements thatwould undergo little change if the measurements were to be repeated. Ithas been found that since the intensity profile distortions associatedwith the second channel sets 510 are predominately systematic andrepeatable, they can be reliably corrected in accordance with variousexample embodiments of the invention.

In step 380, a correction factor is provided for each of the secondchannels sets 510. Since any error in the second intensity valuecorresponding to a given one of the second channel sets 510 arepredominately systematic, the error can be corrected by employing acorrection factor that does not change when employed in a subsequentrecalibration of spatial light modulator 200. In this example embodimentof the invention, the various correction factors can be determined byfitting a mathematical curve to normalized values of the secondintensity values. In this example embodiment, each second intensityvalue is normalized to the average of all the second intensity values.Accordingly, in this example embodiment, each correction factor isdetermined based at least on the average of the all the second intensityvalues as well as the particular second intensity value corresponding tothe second channel set associated with the correction factor. In thisexample embodiment, each correction factor is determined based at leaston a value derived by dividing the second intensity value correspondingto the second channel set associated with the correction factor by theaverage of all of the second intensity values.

Various mathematical curves can be fit to the normalized secondintensity values. For example, FIG. 8 shows three different orthogonalpolynomials fitted to the normalized second intensity values using theGram-Schmidt method. The illustrated polynomials include 3^(rd) degree,5^(th) degree and 9^(th) degree orthogonal polynomials. In particular,5^(th) degree orthogonal polynomials have been employed in the presentinvention with good results. In this example embodiment, each correctionfactor is determined from various points on the mathematical curvecorresponding to a particular second channel set 510. In various exampleembodiments, deriving the calibration factors from a mathematical curvecan be beneficial since the curve performs a substantial averaging ofthe data. Each of the determined correction factors can be stored in acontroller readable memory for a subsequent recalibration of spatiallight modulator 200.

In step 390, each of the intensity values is modified in accordance witha corresponding one of the correction factors. In this exampleembodiment, each of the second intensity values is modified by dividingthe second intensity values by a corresponding one of the correctionfactors. In this example embodiment, each second intensity value ismodified to correct for systematic distortions created in the secondintensity profile as measured by detector 180.

In step 400, an adjustment is performed to reduce a determineddifference between at least one of the modified second intensity valuesand a second target intensity value. In example embodiments where thesecond channel sets 510 are subsets of one or more of the first channelsets 500, this adjustment is referred to as a channel subset adjustment.In some example embodiments, the second target intensity value is equalto one of the modified second intensity values. In some exampleembodiments, the second target intensity value is equal to, or lessthan, a minimum one of the modified second intensity values. Anydetermined difference between a modified second intensity value and thesecond target intensity value is reduced by adjusting control levels ofat least one channel 210 in a corresponding second channel set 510 in adirection appropriate for reducing the determined difference. Since theadjustment of the second channel sets 510 corrects for systematicdistortions in the intensity profile, a subsequent recalibration ofspatial light modulator 200 can be limited to performing the secondchannel set adjustments without performing the adjustment of the firstchannel sets 500.

In some example embodiments, the second target intensity value is thesame as the first target intensity value. In other example embodiments,the second target intensity value is different from the first targetintensity value. In some example embodiments, a single second targetintensity value is compared with each of the modified second intensityvalues. In other example embodiments, different second target intensityvalues are compared with different ones of the modified second intensityvalues. Different second target intensity values can be employed fordifferent reasons including adjusting various channel 210 attenuationlevels in accordance with a specific image pattern feature to be formedas taught in commonly-assigned WO 2008/015515, which is herebyincorporated by reference.

In some example embodiments, all the Y channels 210 in a second channelset 510 are adjusted in accordance with a determined difference betweena corresponding modified second intensity value and a second targetintensity value. In some example embodiments, each of the Y channels 210in a second channel set 510 is proportionally adjusted. Adjustment ofthe control levels of the channels 210 in a given one of the secondchannel sets 510 can include adjusting an attenuation level of thechannels 210. In some example embodiments, some, but not all the Ychannels 210 in a second channel set 510 are adjusted in accordance witha determined difference between a corresponding modified secondintensity value and a second target intensity value.

FIG. 9 schematically represents a plurality of second channel sets 510selected in accordance with another example embodiment of the invention.For clarity, the channels 210 in spatial light modulator 200 areidentified by #1, #2, #3, #4, #5 . . . #(N−1), #N. This numbering schemealso defines the position of each channel 210 in the array of channels210. In this example embodiment, various ones of the second channel sets510 are sub-sets of a first channel set 500.

A first one of the second channel sets 510 (i.e. second channel set510C) is schematically shown comprising a group of adjacent first,second and third channels 210 respectively identified as #1, #2 and #3.An additional or second one of the second channel sets 510 (i.e. secondchannel set 510D) is schematically shown comprising a group of adjacentchannels 210 respectively identified as #2, #3 and #4. In this regard,second channel set 510D “overlaps” second channel set 510C. In thisexample embodiment, several of the channels 210 selected for inclusionin second channel set 510D are also selected for inclusion in secondchannel set 510C. In this example embodiment, adjacent ones of thesecond channels sets 510 such as second channel sets 510C and 510Dinclude at least one common channel 210 and at least one differentchannel 210. In this example embodiment, both of the second channel sets510C and 510D include first channel 210 identified as the #2 channel 210and a second channel 210 identified as the #3 channel 210.

In this example embodiment, each of the second channels set 510 includesY consecutive channels 210 where Y is equal to three (3). In thisexample embodiment, various ones of the second channel sets 510 overlapa adjacent second channel set 510 along arrangement direction of thechannel array by an integer number of consecutive channels 210 numberingless than Y. For example, second channel set 510D has been selected byincrementing the channels 210 selected for second channel set 510C toinclude the #4 channel 210 and by decrementing the second channel set510C to exclude the #1 channel 210.

Accordingly, second channel set 510D overlaps second channel set 210C bytwo (2) channels 210. Other second channel sets 510 are also selected inthis overlapping fashion. For example, second channel set 510E isschematically shown by incrementing the channels 210 selected for secondchannel set 510D to include the #5 channel 210 and by decrementing thesecond channel set 510D to exclude the #2 channel 210. In this exampleembodiment, this process is repeated to select a total of N minus two(2) second channel sets 510. In other example embodiments, the number ofchannels 210 incremented and decremented from a given second channel set510 during the selection of an adjacent second channel set 210 can be asuitable number of channels 210 numbering more than one.

In this example embodiment, each of the second channel sets 510 isseparately operated to provide corresponding output radiation that ismeasured by detector 180 while the control levels of all of the otherchannels 210 belonging to the other second channel sets 510 are set tomaximum attenuation. An intensity value is determined for each outputradiation. For example, a first intensity value (i.e. not to be confusedwith the aforementioned first intensity values provided by the firstchannel sets 500) is determined for first output radiation provided byall of the #1, #2 and #3 channels 210 in second channel set 510C. Asecond intensity value is determined for second output radiationprovided by all of the #2, #3 and #4 channels 210 in the second channelset 510D. In this regard, the output radiation provided by secondchannel set 510D includes output radiation provided by some of thechannels 210 selected for inclusion in the second channel set 510C (i.e.the #2 and #3 channels) and excludes output radiation provided by atleast one of the channels 210 selected for inclusion in the secondchannel set 510C (i.e. the #1 channel). Other intensity values aredetermined for the remaining second channel sets 510 in a similarfashion.

After the first intensity value corresponding to the second channel set510C has been determined, a first adjustment based at least on thedetermined intensity level is performed. In this example embodiment, thefirst adjustment is made after all the various intensity valuesassociated with the second channel sets 510 have been determined. Inother example embodiments, the first adjustment can be made before theoutput radiation corresponding to an additional one of the secondchannel sets 510 is measured. The first adjustment includes adjusting acontrol level of at least one channel 210 in second channel set 510Cwithout adjusting a control level of another channel 210 in secondchannel set 510C. In this example embodiment, the control level of thefirst channel 210 identified as #2 is adjusted, while the control levelof the second channel 210 identified as #3 is not adjusted.

This process is repeated for the other second channels sets 510. Forexample, a second adjustment is performed based at least on the secondintensity level determined for the output radiation provided by thesecond channel set 510D. In a similar fashion, the second adjustmentincludes adjusting a control level of at least one channel 210 in secondchannel set 510D without adjusting a control level of another channel210 in second channel set 510D. In this example embodiment, the controllevel of the second channel 210 identified as #3 is adjusted, while thecontrol level of each of the first channel 210 identified as #2 and thechannel 210 identified as #4 is not adjusted. In this exampleembodiment, the second adjustment includes adjusting the control levelof a channel 210 in second channel set 510D that was previously selectedfor inclusion in second channel set 510C. In this example embodiment,the second adjustment includes adjusting the control level of a channel210 that was previously selected for inclusion in second channel set510C but whose control level was not adjusted by the first adjustment.

In various example embodiments of the invention, each of the adjustmentsis used to reduce a difference between a determined intensity value andtarget intensity value. For example, a first difference between thefirst intensity value corresponding to second channel set 510C and afirst intensity target value can be determined and the first adjustmentcan be performed by adjusting the control level of the #2 channel 210 toreduce the first difference. In a similar fashion, a second differencebetween the second intensity value corresponding to second channel set510D and a second intensity target value can be determined and thesecond adjustment can be performed by adjusting the control level of the#3 channel 210 to reduce the second difference. In some exampleembodiments, the first adjustment can be performed by adjusting thecontrol level of the #2 channel 210 by an amount proportional to thefirst difference while the second adjustment can be performed byadjusting the control level of the #3 channel 210 by an amountproportional to the second difference. In some example embodiments thefirst intensity target value is the same as the second intensity targetvalue while in other example embodiments, the first target intensityvalue is different than the second target intensity value. In someexample embodiments, each of the first adjustment and the secondadjustment are made after both the first difference and the seconddifference have been determined.

In this example embodiment, the first intensity value can be modified inaccordance with a first correction factor and the first adjustment caninclude adjusting the control level of the #2 channel 210 to reduce adifference between the modified first intensity value and a targetintensity value. Likewise, the second intensity value can be modified inaccordance with a second correction factor and the second adjustment caninclude adjusting the control level of the #3 channel 210 to reduce adifference between the modified second intensity value and a targetintensity value. Correction factors including the first and secondcorrection factors can be determined in manners similar to thosedescribed in other example embodiments of the invention.

In this example embodiment, the #2 channel 210 is a centrally locatedbetween two channels 210 in the second channel set 510C and the #3channel 210 is centrally located between two channels 210 in the secondchannel set 510D. In this example embodiment, each of the #2 and the #3channels 210 is adjusted in accordance with total output radiationprovided by all the channels 210 in their respective second channel sets510.

The intensity of an output radiation beam 121 provided by a givenchannel 210 can vary as a consequence of as a function of attenuationlevels of its neighboring channels 210. In particular, the presentinventors have discovered that a measured intensity value of a singleradiation beam 121 provided by a single channel 210 will typically bedifferent than an average intensity value determined from the outputradiation provided by a combined group of the channels 210.Advantageously, the present invention reduces measurement errors byemploying second channel sets 510 that include a plurality of channels210. Further advantageously, the selection of the various “overlapping”second channels sets 510 allows for a greater number of intensity valuesto be determined with channels sets comprising a number of channels 210that is sufficient to reduce neighbor coupling effects. The greaternumber of intensity values allows for a more detailed intensity profileto be determined. In this sense, the array of channels 210 is sampledwith a higher sampling addressability than the resolution of the samplesthemselves.

A program product can be used by controller 30 to perform variousfunctions required by recording apparatus 10. One such function caninclude calibrating a spatial light modulator 200 in accordance with amethod or combination of methods taught herein. Without limitation, theprogram product may comprise any medium which carries a set ofcomputer-readable signals comprising instructions which, when executedby a computer processor, cause the computer processor to execute amethod as described herein. The program product may be in any of a widevariety of forms. The program product can comprise, for example,physical media such as magnetic storage media including, floppydiskettes, hard disk drives, optical data storage media including CDROMs, DVDs, electronic data storage media including ROMs, flash RAM, orthe like. The instructions can optionally be compressed and/or encryptedon the medium.

It is to be understood that the exemplary embodiments of the inventionare merely illustrative and that many variations of the describedembodiments can be devised by those skilled in the art without departingfrom the scope of the invention. In this regard, it is to be understoodthat various aspects of one or more of the example embodiments can becombined with aspects of other example embodiments without departingfrom the scope of the present invention.

PARTS LIST

-   10 recording apparatus-   12 media support-   13 cylindrical surface-   16 recording head-   17 recording media-   18 carriage-   19 image-   20 support-   22 motion system-   25 registration features-   28 clamps-   30 controller-   32 guide system-   33 transmission member-   37 image data-   100 optical system-   102 illumination source-   110 optical element(s)-   121 output radiation beams-   125 radiation-   140 image pixel-   150 aperture-   170 optical element(s)-   180 detector-   182 non-sampling position-   184 sampling position-   185 actuator system-   200 spatial light modulator-   210 channel-   212 member-   215 electrode-   216 electrode-   218 surface-   220 surface-   222 surface-   300 method-   310 select a plurality of first channel sets-   320 determine a first intensity value for each first channel set-   330 generate a first intensity profile from the determined first    intensity values-   340 reduce a determined difference between at least one of the first    intensity values and a first target intensity value-   350 select a plurality of second channel sets-   360 determine a second intensity value for each second channel set-   370 generate a second intensity profile from the determined second    intensity values-   380 provide a correction factor for each second intensity value-   390 modify each second intensity value in accordance with a    corresponding correction factor-   400 reduce a determined difference between at least one of the    modified second intensity values and a second target intensity value-   450 multi-value detector intensity profile-   460 first single-value detector intensity profile-   470 second single-value detector intensity profile-   500 first channel sets-   510 second channel sets-   510A second channel set-   510B second channel set-   510C second channel set-   510D second channel set-   510E second channel set

The invention claimed is:
 1. A method for calibrating a spatial lightmodulator comprising an array of individually addressable channels, themethod comprising: selecting a plurality of channel sets from the arrayof channels, each channel set comprising a plurality of channels;operating each of the channel sets to provide corresponding first outputradiation; determining a plurality of intensity values, each of theintensity values corresponding to the first output radiation provided bya different one of the channel sets; selecting a first intensity valuefrom the plurality of intensity values, wherein the first intensityvalue corresponds to a first channel set in the plurality of channelsets; determining a difference between the first intensity value and afirst target intensity value; performing a channel set adjustment, thechannel set adjustment comprising adjusting a control level of eachchannel in the first channel set to reduce the determined differencebetween the first intensity value and the first target intensity value;selecting a plurality of channel subsets from the first channel set;providing a correction factor for each channel subset; and performing achannel subset adjustment, the channel subset adjustment comprisingadjusting a control level of at least one channel in each of at leastone of the channel subsets in accordance with a corresponding one of thecorrection factors.
 2. A method according to claim 1, wherein thechannel subset adjustment is performed after performing the channel setadjustment.
 3. A method according to claim 2, comprising employing eachof the correction factors in a subsequent recalibration of the spatiallight modulator, wherein each correction factor remains unaltered duringthe subsequent recalibration of the spatial light modulator.
 4. A methodaccording to claim 3, wherein the channel subset adjustment is performedwithout performing the channel set adjustment during the subsequentrecalibration of the spatial light modulator.
 5. A method according toclaim 1, wherein the channel subset adjustment comprises: selecting afirst channel subset from the plurality of channel subsets; operatingthe first channel subset to emit second output radiation; determining asecond intensity value for the second output radiation; modifying thesecond intensity value in accordance with the correction factorcorresponding to the first channel subset; determining a differencebetween the modified second intensity value and a second targetintensity value; and performing the channel subset adjustment on thefirst channel subset to reduce the determined difference between themodified second intensity value and the second target intensity value.6. A method according to claim 5, wherein the first target intensityvalue is equal to one of the plurality of intensity values other thanthe first intensity value.
 7. A method according to claim 5, wherein thefirst target intensity value is determined by a distribution of theplurality of intensity values.
 8. A method according to claim 5, whereinthe second target intensity value is equal to the first target intensityvalue.
 9. A method according to claim 1, wherein each of the channelsets is operated separately to provide corresponding first outputradiation.